In recent years, from the viewpoint of conserving energy, tires having a small rolling resistance are demanded. For this reason, as a rubber composition used in a tire tread and the like, a rubber composition having a low tan δ and excellent low-heat-generation property is demanded. Further, the rubber composition for use in a tread is required to have excellent abrasion resistance as well as excellent low-heat-generation property. For meeting such demands, in order to improve the low-heat-generation property and abrasion resistance of the rubber composition, various attempts are made to improve the affinity of a filler, such as carbon black or silica, with a rubber component in the rubber composition.
For example, PTL 1 has proposed a modified natural rubber which is obtained by adding a polar group-containing compound to at least one natural rubber raw material selected from the group consisting of natural rubber, a natural rubber latex coagulation product, and a natural rubber cup lump, and applying a mechanical shear force to the resultant mixture to cause graft polymerization or addition.
PTL 2 discloses a modified natural rubber latex which is obtained by adding a polar group-containing mercapto compound to a natural rubber latex so that the polar group-containing mercapto compound is added to natural rubber molecules in the natural rubber latex.
Further, PTLs 3 to 6 have proposed a modified natural rubber which is obtained by graft-polymerizing various polar group-containing monomers on a natural rubber latex.
However, for obtaining a rubber component which has a further improved affinity with a reinforcing filler formed from carbon black and/or silica so as to enable the rubber composition to be further improved in low-heat-generation property and abrasion resistance, the modified conjugated diene polymer is needed to be further improved.